Various graphitic materials are commonly employed as an active material for negative electrodes in lithium-ion batteries in view of their high negative redox potential, their high specific electrochemical capacity for lithium insertion, low first cycle capacity loss, and good cycle life. On the other hand, graphite generally exhibits only low volumetric density, a high sensitivity to electrolytes, and is furthermore prone to undesirable exfoliation.
Lithium-ion cells typically operate under conditions which lead to decomposition of the organic electrolytes, where the decomposition products form a protective film at the carbon-electrolyte interface. This protective film commonly referred to as solid electrolyte interphase (SEI) will ideally act as an electronically insulating layer, thereby preventing continued electrolyte decomposition while still allowing the transport of lithium ions. It is generally understood that the transport of lithium ions during charge/discharge cycles occurs on the prismatic rather than the basal plane surfaces of the graphite particles (see, for example, Placke et al., J. of Power Sources 200 (2012), pp. 83-91).
The SEI formation typically occurs in the first few charge/discharge cycles of the lithium ion cell operation, although it also affects the long-term cycle life. In any event, the SEI formation is connected with an irreversible consumption of lithium and electrolyte material, which in turn leads to an irreversible charge loss commonly referred to as “irreversible capacity” (Cirr).
Because the loss of lithium (and decomposition of electrolytes) reduces the specific capacity of the cell, attempts have been made to optimize the SEI layer formation in order to reduce the irreversible capacity while still forming an effective, but thin SEI. In general, it is believed that the SEI formation largely depends on the electrode surface morphology which is in contact with the electrolyte. Factors affecting the formation of the SEI include, amongst others, the type of binder and the porosity of the electrode. For negative electrodes wherein the active material is graphite, the type of graphite (e.g., particle size distribution, particle shape or morphology, surface area, functional groups on the surface, etc.) also appears to influence the SEI layer formation.
Surface-modified graphites have been described in the art, wherein the modification of the surface aimed at the optimization of the surface properties of the modified graphite during SEI layer formation in order to improve cycle life, reversible discharge capacity and irreversible capacity.
For example, the surface of natural or synthetic graphite has been modified by treatments contacting natural graphite with a concentrated sulfuric acid solution at high temperatures (210° C.), followed by coating with a resin and subsequent heat treatment for 3 h at 800° C. in a nitrogen atmosphere (Zhao et al., Solid State sciences 10 (2008), pp. 612-617, CN101604750). Others have described the heat treatment of potato shape graphite (TIMREX® SLP30) in oxygen atmosphere (Plancke et al., Journal of Power Sources, 200 (2012), pp. 83-91), or the treatment of synthetic graphite (LK-702 Nippon Carbon) in air atmosphere for very long residence times (6 to 56 h) (Rubino et al., Journal of Power Sources 81-82 (1999), pp. 373-377). TIMCAL published that oxidation of heat-treated ground graphite could suppress the exfoliation of graphite observed in ethylene carbonate based electrolyte systems (Journal of the Electrochemical Society, 149(8) (2002), A960-A966, Journal of The Electrochemical Society, 151(9) (2004), pp. A1383-A1395, Journal of Power Sources 153 (2006), pp. 300-311). Goers et al, Ionics 9 (2003), pp. 258-265 treated synthetic graphite (TIMREX® SLX 50) for 1 hour at various temperatures with air, observing an increase in the crystallite size La (determined by Raman spectroscopy) after the oxidation treatment. Contescu et al. (Journal of Nuclear Materials 381 (2008), pp. 15-24) examined the effect of air flow rate on the properties of various surface-oxidized 3-dimensional graphite specimens (i.e. graphite bars) including a binder material, reporting inter alia that the intensity of the D band decreased compared to the G band after oxidation treatment, indicating an increase in the surface order of the treated graphite particles.
The prior art also reported on alternative surface modification treatments wherein graphite particles were coated with carbon by a technique called chemical vapor deposition (CVD). For example, Guoping et al., Solid State Ionics 176 (2005), pp. 905-909, describe the coating of milled spherical natural graphite by CVD at temperatures of between 900 and 1200° C. leading to improved initial coulombic efficiency and better cycle stability. Liu et al., New Carbon materials 23(1) (2008), pp. 30-36 likewise describe natural graphite materials modified by CVD (coating with acetylene gas at 1000° C.) with a disordered carbon structure (MNGs), which are said to exhibit improved electrochemical properties over natural graphite. Park et al., Journal of Power Sources 190 (2009), pp. 553-557, examined the thermal stability of CVD coated natural graphite when used as an anode material in lithium-ion batteries. The authors found that carbon coating of natural graphite by CVD led to a lower irreversible capacity and increased coulombic efficiency compared to unmodified natural graphite. Natarajan et al., Carbon 39 (2001), pp. 1409-1413, describe the CVD coating of synthetic graphite at temperatures between 700 and 1000° C. The authors report that a CVD coating at around 800° C. yielded the best results in terms of coulombic efficiency while showing a decreased disorder of the treated graphite particles (i.e. the intensity of the D band as determined by Raman spectroscopy decreases compared to the untreated material). Interestingly, the authors report that at 1000° C. the intensity of the D band increased, hinting at an increased disorder on the surface of the graphite particles at the higher temperature. Finally, Ding et al., Surface & Coatings Technology 200 (2006), pp. 3041-3048, likewise report on CVD coated graphite particles by contacting synthetic graphite with methane at 1000° C. Ding et al. conclude that the graphite particles coated by CVD for 30 minutes at 1000° C. exhibited improved electrochemical properties compared to untreated graphite material.
Overall, it appears that the results reported in the prior art remain somewhat inconclusive with regard to the desirable parameters of the surface-modified graphite particles as well as the process parameters for obtaining favorable surface-modified graphite materials exhibiting improved electrochemical properties. It is therefore an object of the invention to provide alternative surface-modification processes for synthetic graphitic carbons which yield surface-modified graphite materials having excellent physicochemical as well as electrochemical properties, especially when used as a material for negative electrodes in lithium-ion batteries. Thus, another related object of the present invention is to provide alternative surface-modified graphite materials generally having the aforementioned favorable physicochemical and electrochemical properties, particularly when used in lithium-ion batteries.